1. Field of the Invention
The present invention relates generally to contamination inspection for semiconductor wafers and the like and in particular to a system which inspects both the frontside and backside of a semiconductor wafer without manual or automatic inversion of the wafer.
2. Description of the Related Art
Tools used in the semiconductor wafer manufacturing process must periodically be checked to determine whether they must be replaced or are still in usable condition. The condition of a tool is checked by inspecting wafers processed by that tool for defects. Bare wafers are typically routed through the process tool with the frontside facing up, and wafer defects detected optically by illuminating portions of the wafer and measuring the amount of illuminating light scattered by defects on the wafer surface.
Previously, systems which performed inspection of wafers did so in two discrete stages. First, the frontside of the wafer was scanned for contamination caused by the process tool. If the defect rate on the frontside of the wafer was acceptable, the wafer was then turned over to inspect the backside for further particle contamination and other defects. The process tool was considered usable if the defect rate on the backside of the wafer was also acceptable.
Inspection of both sides of a wafer by these procedures accordingly required time for inspection of one side, examination of the one side, inverting the wafer without excessively damaging the wafer, scanning the reverse side, and examining the results of the second side scan. In addition to this excessive amount of time required for examination, the process of flipping the semiconductor wafer had a tendency to contaminate the edges of the wafer due to surface or edge contact with a gripping device. In some processes, when the wafer was flipped over to inspect the backside, the front side of the wafer could be contaminated by the flipping process. The resulting contamination of the frontside of the wafer tends to render the wafer unsuitable for further processing. Thus, all test wafers were usually scrapped after each inspection, reducing overall productivity and increasing per unit cost.
Edge handling of wafers has also complicated the problem. As wafers tend to suffer from contamination or other degradation when handled by wafer orientation systems, the handling of a wafer requires special care. Although previous wafer orientation systems have included multiple drive rollers, radially inwardly-biased contact rollers, and a tiltable wafer-supporting table with an air-bearing mechanism, each of these handling methods have benefits and drawbacks. Systems without multiple drive rollers and radially inwardly-biased or spring-loaded contact rollers cannot maintain steady wafer rotation rate during the portion of a cycle in which the drive roller is not in contact with the round edge of the wafer because the drive roller loses traction along the wafer edge.
In inspection equipment, it is important to maintain steady rates of wafer rotation to avoid errors in defect detection, such as errors in detecting defects where none exist, or simply failing to detect defects. Previous systems which supported semiconductor wafers through direct contact with a solid surface present special problems during inspection since contact with the support surface may increase contamination or move defects from one location to another in ways that render the wafer unsuitable for future processing.
It is therefore an object of the current invention to provide a system for minimizing the time required for full inspection of both the front side and back side of a wafer.
It is another object of the current invention to provide an arrangement which minimizes overall wafer contamination during the inspection process, particularly when inspecting both front and back sides of the wafer.
It is a further object of the current invention to minimize edge handling concerns, such as contamination, during the inspection of the front side and back side of a wafer.
It is still a further object of the current invention to minimize the number of defects missed or falsely detected by the inspection system.
According to the present invention, there is provided an apparatus that simultaneously inspects the frontsides and backsides of semiconductor wafers for defects. The inventive system disclosed herein may also read tracking information imprinted on the backsides of the semiconductor wafers.
The invention rotates the semiconductor wafer while the frontside and backside surfaces are generally simultaneously optically scanned for defects. Rotation is induced by providing contact between the beveled edges of the semiconductor wafer and roller bearings rotationally driven by a motor.
In the present invention, a semiconductor wafer is supported such that the semiconductor wafer lays flat during the inspection process. The surface is large enough to accommodate the wafer as well as the rollers for rotating the wafer and the means for holding the wafer. The wafer is preferably supported in a tilted or semi-upright orientation such that support is provided by gravity. This tilted supporting orientation permits both the frontside and the backside of the wafer to be viewed simultaneously by a frontside inspection device and a backside inspection device. The backside of the wafer for purposes of this invention is the side of the semiconductor wafer by which the wafer is being supported. Simultaneous dual-side inspection of the front side and back side of the wafer effectively doubles the throughput of inspection equipment and eliminates the need to turn the semiconductor wafer over during the inspection process, thereby reducing the opportunity for edge contamination of the inspected wafer.
The wafer is rotated by multiple motor-driven roller, bearings. These drive rollers are positioned at the circumference of the wafer and are angled such that the roller pads contact the wafer only along the beveled edge. This periphery positioning and rotation coupled with angular contact between the rollers and wafer edge and surface permits inspection of the entire surface and significantly reduces the potential for contamination of the surface resulting from edge contact, or contact with the roller pads.
The drive rollers are spaced apart such that at least one of the two drive rollers spaced farthest apart contacts the round edge of the wafer throughout the rotation cycle. This constant contact feature ensures that the rotation rate of the wafer is suitably steady during defect inspection. Also, the steady rotation rate minimizes the number of defects missed or falsely detected by the inspection system.
The wafer rotation rate is such that roller contact does not damage the wafer edge. Furthermore, defects are not carried or transported from one part of the edge to another. Moreover, the rate should be controlled so as to minimize slip between the roller and the wafer edge. The present invention is intended for use at wafer rotation rates on the order of 400 revolutions per minute. Unlike previous systems, the present invention does not exhibit excessive vibration for defect inspection purposes at these rotation rates. The increased wafer rotation rate also increases the throughput of inspection equipment.
The semiconductor wafer is held against the drive rollers by pressure using a set of undriven roller bearings (contact rollers) or alternatively simply using gravitational force by tilting the wafer and inspection surface. This pressure ensures that the drive rollers hold traction on the beveled wafer edge so that a steady rotation rate can be maintained. All contact rollers thereby maintain contact with the edge of the semiconductor wafer throughout the rotation cycle.
Prior to inspection, the system locates the edge registration feature, commonly called the xe2x80x9cflatxe2x80x9d. The system detects the specific position of the wafer using the edge registration feature either by measuring the position of the contact rollers, or by connecting the contact rollers to switches which are turned on when the contact rollers are touching a flat registration edge calibration switch. Once the flat registration edge or notch is located, the system rotates the wafer to desired orientations for inspection purpose by controlling the drive rotors.